Firefox 24.0 - really?

How about using a newer version of Firefox?

Old Firefox versions 45.0 and earlier used Gstreamer while 46.0 and later including the current 61.0.2 Release and 60.1.0esr uses FFmpeg to provide the needed codecs to fully use the HTML5 players for video and audio.

You can see what you support at say https://www.youtube.com/html5 as all checks should be blue.

Ther was only 2 blue checks out of 6. THE h.264 wasnt blue.

https://support.mozilla.org/en-US/kb/switch-to-firefox-extended-support-release-esr

Verify that you meet the System Requirements (GTK+ and GLib) for the current Firefox release.

• https://www.mozilla.org/en-US/firefox/system-requirements/
• https://www.mozilla.org/en-US/firefox/61.0.1/system-requirements/

www.mozilla.org/firefox/all/

The Firefox 24.0 and Ubuntu in useragent makes me think the OS is the old Ubuntu 13.10 (or flavour of) from Oct 2013 and never updated Firefox in package manager. http://distrowatch.org/table.php?distribution=ubuntu

Yes one big requirement of late is you need GTK 3.4 (three.four) at minimum) for Firefox 46.0 and later including 61.0.2 to run though most not LTS versions of Linux distros in past few years likely have at least 3.14 (three.fourteen) and the current is on 3.22.x (three.twenty-two) branch.